Cathode active material and lithium secondary battery comprising same

Abstract

Disclosed is a cathode active material for secondary batteries in which a carboxymethyl cellulose derivative is coated on surfaces of particles of a lithium transition metal oxide having the formula Li.sub.xM.sub.yO.sub.2 where M: Ni.sub.aMn.sub.bCo.sub.c wherein 0≦a≦0.9, 0≦b≦0.9, 0≦c≦0.5, and 0.85≦a+b+c≦1.05 and x+y=2, wherein 0.95≦x≦1.15.

Claims

1. A cathode mixture for secondary batteries, comprising: a cathode active material and a binder, wherein the cathode active material, in which a carboxymethyl cellulose derivative is coated on surfaces of particles of a lithium transition metal oxide represented by Formula 1 below:
Li.sub.xM.sub.yO.sub.2  <Formula 1> wherein M is Ni.sub.aMn.sub.bCo.sub.c where 0≦a≦0.9, 0≦b≦0.9, 0≦c≦0.5, and 0.85≦a+b+c≦1.05); and x+y=2, wherein 0.95≦x≦1.15, wherein the carboxymethyl cellulose derivative is a material represented by Formula 2 below: ##STR00002## wherein at least one selected from the group consisting of R.sub.1, R.sub.2, and R.sub.3 is CH.sub.2CO.sub.2X and the remaining group is CH.sub.2CO.sub.2H or OH, wherein X is Na; and n is an integer of 1 or more and is determined within a range within which the carboxymethyl cellulose derivative has a weight average molecular weight (Mw) of 1,700,000 to 2,300,000, wherein a rate to which R.sub.1, R.sub.2, and R.sub.3 are CH.sub.2CO.sub.2X is 0.8 to 1.2, wherein the binder is styrene-butadiene rubber, wherein an amount of the binder is 1 wt % to 20 wt % based on a total weight of the cathode mixture, and wherein a weight ratio of the carboxymethyl cellulose derivate:the styrene-butadiene rubber is 0.5:0.8 to 0.7:1.0.

2. The cathode mixture according to claim 1, wherein, in Formula 1, amounts of Ni, Mn and Co satisfy the conditions: 0.2≦a≦0.7, 0<b≦0.5, and 0<c≦0.5.

3. The cathode mixture according to claim 1, wherein M is substituted with at least one element selected from the group consisting of Al, Mg, Fe, Ti, V, Zr, Sn, and Zn.

4. The cathode mixture according to claim 1, wherein a coating amount of the carboxymethyl cellulose derivative is in a range of 0.1 wt % to 10 wt % based on a total weight of the cathode active material.

5. The cathode mixture according to claim 1, further comprising at least one material selected from the group consisting of a dispersion medium, a conductive material, a viscosity modifier, a filler, a coupling agent, and an adhesion promoter.

6. A cathode for secondary batteries, in which the cathode mixture according to claim 1 is coated on a current collector.

7. A lithium secondary battery comprising the cathode according to claim 6.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a graph showing electrochemical impedance spectra of cathodes for lithium secondary batteries fabricated according to Example 1 and Comparative Example 2, obtained according to Experimental Example 1;

(3) FIG. 2 is a graph showing discharge capacities at 0.2 C and 1 C of the cathodes of Example 1 and Comparative Example 2, obtained according to Experimental Example 2;

(4) FIG. 3 is a graph showing lifespan characteristics under high temperature and high voltage conditions of rectangular-type lithium batteries manufactured using the cathodes of Example 1 and Comparative Example 2; and

(5) FIG. 4 is a graph showing lifespan characteristics under high temperature and high voltage conditions of rectangular-type lithium batteries manufactured using cathodes for lithium secondary batteries fabricated according to Example 1 and Comparative Example 4.

MODE FOR INVENTION

(6) Now, the present invention will be described in more detail with reference to the accompanying drawings and the following examples. These examples are only provided for illustration of the present invention and should not be construed as limiting the scope and spirit of the present invention.

Example 1

(7) LCO as a cathode active material, carbon black as a conductive material, and a binder (Na-based CMC having a molecular weight of 1,700,000: SBR=0.5:0.8) were added to NMP in a weight ratio of 96.95:1.75:1.3 and mixed to prepare a cathode mixture. Subsequently, the cathode mixture was coated on Al foil having a thickness of 20 μm to a thickness of 200 μm and the coated Al foil was rolled and dried, thereby completing fabrication of an electrode.

(8) The electrode is subjected to blanking into a coin shape, and the coin-shaped electrode, graphite as an anode, and a carbonate solvent containing 1 M LiPF.sub.6 as an electrolyte were used to manufacture a coin-type battery.

Example 2

(9) A lithium secondary battery was manufactured in the same manner as in Example 1, except that LCO as a cathode active material and a binder (Na-based CMC: SBR=0.7:0.8) were added in a weight ratio of 96.75:1.5.

Example 3

(10) A lithium secondary battery was manufactured in the same manner as in Example 1, except that LCO as a cathode active material and a binder (Na-based CMC: SBR=0.7:1.0) were added in a weight ratio of 96.55:1.7.

Example 4

(11) A lithium secondary battery was manufactured in the same manner as in Example 1, except that Na-based CMC having a molecular weight of 2,200,000 was used.

Comparative Example 1

(12) A lithium secondary battery was manufactured in the same manner as in Example 1, except that 1.5 wt % of PVdF as a binder was added.

Comparative Example 2

(13) A lithium secondary battery was manufactured in the same manner as in Example 1, except that 2 wt % of PVdF as a binder was added.

Comparative Example 3

(14) A lithium secondary battery was manufactured in the same manner as in Example 1, except that Na-based CMC having a molecular weight of 300,000 was used.

Comparative Example 4

(15) A lithium secondary battery was manufactured in the same manner as in Example 1, except that Na-based CMC having a molecular weight of 3,000,000 was used.

(16) TABLE-US-00001 TABLE 1 Cathode Conductive PVdF CMC (average Electrode material material (binder) molecular weight) SBR adhesion (gf) Comparative 96.75 1.75 1.5 0 0 30 Example 1 Comparative 96.25 1.75 2 0 0 45 Example 2 Comparative 96.95 1.75 0 0.5 (0.3 million) 0.8 9 Example 3 Comparative 96.95 1.75 0 0.5 (3 million).sup.  0.8 120 Example 4 Example 1 96.95 1.75 0 0.5 (1.7 million) 0.8 45 Example 2 96.75 1.75 0 0.7 (1.7 million) 0.8 50 Example 3 96.55 1.75 0 0.7 (1.7 million) 1.0 65 Example 4 96.95 1.75 0 0.5 (2.2 million) 0.8 85 (Amount: wt %)

Experimental Example 1

(17) Electrochemical impedance spectroscopy was performed using each of the lithium secondary batteries manufactured according to Example 1 and Comparative Example 2.

Experimental Example 2

(18) The lithium secondary batteries of Example 1 and Comparative Example 2 were charged to a high voltage of 4.4 V and then discharge capacities at 0.2 C and 1 C thereof were measured.

Experimental Example 3

(19) Lifespan characteristics under high temperature and high voltage conditions were compared between the lithium secondary batteries of Example 1 and Comparative Example 2.

Experimental Example 4

(20) Lifespan characteristics under high temperature and high voltage conditions were compared between the lithium secondary batteries of Example 1 and Comparative Example 4.

(21) Results of Experimental Examples 1 to 4 are illustrated in FIGS. 1 to 4.

(22) First, referring to FIG. 1, the lithium secondary battery of Example 1 in which SBR was used as a binder and surfaces of particles of the cathode active material were coated with a carboxymethyl cellulose derivative and the lithium secondary battery of Comparative Example 2 using PVdF as a binder were subjected to electrochemical impedance spectroscopy. As illustrated in FIG. 1, it can be confirmed that the lithium secondary battery of Example 1 had a higher interfacial resistance due to formation of a cathode film through coating with CMC than the lithium secondary battery of Comparative Example 2.

(23) Referring to FIG. 2, it can be confirmed that the lithium secondary batteries of Example 1 and Comparative Example 2 exhibit similar performance during discharge at 0.2 C, while the lithium secondary battery of Example 1 exhibits superior performance to that of the lithium secondary battery of Comparative Example 2 in a high-rate discharging process, i.e., discharge at 1 C.

(24) Referring to FIG. 3, it can be confirmed that, as charging and discharging continue, the lithium secondary battery of Comparative Example 2 using PVdF as a binder exhibits dramatically deteriorated cycle characteristics as compared to the lithium secondary battery of Example 1 in which SBR was used as a binder and surfaces of particles of the cathode active material were coated with a carboxymethyl cellulose derivative.

(25) Referring to FIG. 4, it can be confirmed that the lithium secondary battery of Comparative Example 4 using CMC having a molecular weight of 3,000,000 has high electrode adhesion, while exhibiting dramatically deteriorated cycle characteristics as compared to the lithium secondary battery of Example 1 using CMC having a molecular weight of 1,700,000, as charging and discharging continue.

INDUSTRIAL APPLICABILITY

(26) As described above, in a lithium secondary battery using a cathode active material according to the present invention, particles of which are coated with a carboxymethyl cellulose derivative, the carboxymethyl cellulose derivative serves as a thickening agent and facilitates dispersion of the cathode active material, and forms a film at a cathode surface and thus prevents elution of metals, which results in enhanced lifespan characteristics at high voltage and high temperature.

(27) Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.